1. The Field of the Invention
The invention relates to orthopaedics, and more particularly, to systems and methods for treatment for the cervical or thoracolumbar spine that embody both principles of providing motion restoration as well as balance control.
2. The Relevant Technology
Cervical spondylosis is an almost universal concomitant of human aging. More than half of middle-age populations have radiographic or pathologic evidence of cervical spondylosis. Spondylosis with resulting cord compression is the pathogenic factor in 55% of cervical myelopathy cases. The exact pathophysiology of cervical spondylotic myelopathy (CSM) remains unclear. Some proposed mechanisms include direct mechanical compression, microtrauma and ischemia to the cervical spinal cord.
A variety of factors have been implicated as predictors of clinical outcome following surgery. These include age, duration of symptoms prior to surgery, severity of myelopathy before surgery, multiplicity of involvement, anteroposterior canal diameter, transverse area of the spinal cord and high-signal intensity area on T2-weighted imaging.
Surgery is reserved for patients with a progressive history of worsening signs or symptoms, severe spinal cord compression found on imaging studies and failure to respond to non-operative treatment. Operative treatment is directed at relieving the spinal cord compression by expanding the spinal canal diameter. Surgical options include anterior discectomy and fusion (ACDF), corpectomy, laminectomy with or without fusion, and laminoplasty. The choice of an anterior or posterior approach to decompression is influenced by several factors: the degree of disc herniation, osteophyte formation, ligamentous hypertrophy, facet degeneration, number of levels involved, spinal alignment and mobility must all be taken into consideration. A relative indication for an anterior approach, including corpectomy or cervical discectomy and fusion, is the pre-operative presence of cervical kyphosis or straightening of cervical spine. In such circumstances, an anterior single or multilevel approach restores the alignment of the anterior and middle columns, avoiding post-laminectomy progression of kyphosis with worsening deficit. However, multi-level anterior procedures may be associated with significant risks and potential complications. In the setting of myelopathy secondary to multilevel posterior disease, particularly in the elderly, a posterior approach may be more appropriate.
For patients with a neutral to lordotic cervical alignment, laminoplasty has been advocated as an alternative to laminectomy and fusion or multi-level corpectomy. Laminoplasty has the theoretical advantage of preserving spinal motion. Unfortunately, laminoplasty is not indicated in the setting of preoperative cervical straightening or kyphosis. In the setting of straightening, pre-operative kyphotic deformity or degenerative spondylosis in the subaxial spine, laminectomy alone has been implicated in the development iatrogenic post-laminectomy kyphosis. Removal of the interspinous ligaments, ligamentum flavum along with devascularization of the paravertebral muscles has been implicated in the loss of the “posterior tension band” in decompression cases. Unfortunately, multi-level decompression and fusion can be associated with significant loss of range of motion for the subaxial cervical spine. In addition, multilevel fusion can be associated with significant risks for adjacent segment degeneration.
Laminectomy remains a mainstay of surgical decompression for multi-level CSM. However, drawbacks include the risks of post-laminectomy kyphosis, instability, accelerated spondylotic changes, and late neurological deficit. Post-laminectomy kyphosis is twice as likely to develop if there is preoperative loss of the normal cervical lordosis. Laminectomy with concomitant posterolateral fusion has been advocated as a means of attaining neural decompression while avoiding iatrogenic kyphosis. Fusion has, however, the disadvantage of converting a functionally mobile, mechanically stable spinal unit into a fixed, nonfunctional one. Analysis of strain distribution in intervertebral discs following fusion has shown an increase in longitudinal strain, most commonly at levels immediately adjacent to the fused segments. The resultant increase in stress on discs adjacent to the fused levels is thought to lead to accelerated disc degeneration and/or mechanical instability at adjacent levels. Radiographic changes of spondylosis and instability at levels above and below cervical fusions have been described by several authors. No motion sparing surgical solution currently exists for these patients. Therefore, a need exists for technology that allows reconstitution of the posterior tension band following decompression with laminectomy.